In a power amplifier, an electron tube, or the like, which consumes high power, the temperature of the apparatus needs to be monitored. Moreover, if the apparatus temperature rises to a predetermined value, the power amplifier, an electron tube, or the like need to be cooled using a fan or the like or the operation thereof needs to be stopped. Thus, these electronic apparatuses need to be prevented from being damaged.
Conventional methods for detecting the temperature of the electronic apparatus involve a thermostat and a thermistor. In the above-described power amplifier and electron tube, a thermostat or a thermistor is attached to an electrode of an amplifying transistor, an electrode of the electron tube, or the like which serve as a heating element, for thermal coupling.
However, the amplifying transistor, electron tube, or the like often operate at a high voltage. Thus, when the thermostat or the thermistor is attached directly to such a component, if the attachment site includes an electrical conductor, high voltage is applied to the thermostat or the thermistor.
In general, a detecting circuit uses a thermostat or a thermistor to output a temperature detection signal, an alarm signal, and the like. The detecting circuit operates at a relatively low voltage in order to supply output signals to a logical circuit and the like.
Thus, to allow the temperature of a heating element operating at a high voltage to be measured, the heating element needs to be insulated from the thermostat or the thermistor, to which the high voltage is applied, needs to be insulated from the detecting circuit, which uses the thermostat or the thermistor to output a temperature detection signal, an alarm signal, and the like.
The temperature detection apparatus in the background art adopts the technique of, for example, as shown in FIG. 1, attaching thermostat 202 to heating element 200 via insulator 201 to insulate heating element 200 from thermostat 202. FIG. 1 shows an example in which thermostat 202 is used to detect the temperature. However, the above-described technique also applies to the use of a thermistor.
Japanese Patent Laid-Open No. 7-49720 describes a configuration in which a detecting circuit (referred to as a “temperature sensing circuit” in Japanese Patent Laid-Open No. 7-49720) is operated by being supplied with the same high voltage as that applied to a heating element, thus eliminating the need to insulate a thermostat from the detecting circuit.
In the above-described background art, the configuration shown in FIG. 1 and in which the insulator is interposed between the heating element and the thermostat, the thermostat detects the temperature of the heating element via the thermal resistance of the insulator. Thus, accurately measuring the temperature of the heating element is difficult. Furthermore, sensing of the temperature may disadvantageously be delayed (time lag).
On the other hand, in the configuration described in Japanese Patent Laid-Open No. 7-49720, the detecting circuit is operated by being supplied with the same high voltage as that applied to the heating element. Thus, output signals from the detecting circuit cannot directly be supplied to a logical circuit or the like which operates at a relatively low voltage (hereinafter referred to as a low-voltage operating circuit). Consequently, the detecting circuit needs to be insulated from the low-voltage operating circuit that receives output signals from the detecting circuit or it also needs to carry out required processing (control of a fan and control such as the stop of the operation of the circuit including the heating element) using signals detected by the thermostat. As a result, the configuration of the detecting circuit is disadvantageously complicated.